Stel­lar in­ter­lop­ers could be speed­ing towards us at huge ve­loc­ity

Sky at Night Magazine - - BULLETIN -

A group of as­tronomers look­ing for stars es­cap­ing the Milky Way could in­stead have found sev­eral in­vad­ing it from an­other galaxy.

The team made the dis­cov­ery dur­ing a re­cent anal­y­sis of data from the Gaia space­craft. Since 2014, Gaia has mea­sured the po­si­tions, par­al­laxes (a mea­sure of dis­tance) and 2D proper motion on the sky of over a bil­lion stars. Seven mil­lion of these stars have full 3D data, mean­ing it’s pos­si­ble to work out how quickly they are mov­ing towards or away from us.

A team of re­searchers from Lei­den Uni­ver­sity in the Nether­lands used this 3D data to search for hy­per­ve­loc­ity stars. These are stars that have been ac­cel­er­ated to phe­nom­e­nal speeds, prob­a­bly through in­ter­ac­tions with a su­per­mas­sive black hole.

“Of the seven mil­lion Gaia stars with full 3D ve­loc­ity mea­sure­ments, we found 20 that could be trav­el­ling fast enough to even­tu­ally es­cape the Milky Way,” says Elena Maria Rossi, one of the re­searchers in­volved in the study.

Based on a pre­vi­ous look at the Gaia data, the team ex­pected to find per­haps one hy­per­ve­loc­ity star break­ing loose from the Galaxy, so were pleas­antly sur­prised when so many turned up. More un­ex­pected was the di­rec­tion in which the stars were trav­el­ling.

“Rather than fly­ing away from the Ga­lac­tic cen­tre, most of the high ve­loc­ity stars we spot­ted seem to be rac­ing towards it,” says Tom­maso Marchetti, who took part in the re­search. “These could be from an­other galaxy, zoom­ing right through the Milky Way.”

The stars may have orig­i­nated in the Large Mag­el­lanic Cloud or they could come from much fur­ther afield. As­tronomers will now study the stars to see if they can de­ter­mine their ori­gin. If they do turn out to be in­ter­ga­lac­tic in­ter­lop­ers, the stars could of­fer re­searchers a unique op­por­tu­nity to take a de­tailed look at stars from an­other galaxy.

“Look­ing at the colours of the stars tells us what they are made of,” says Marchetti. “A star from the Milky Way halo is likely to be fairly old and mostly hy­dro­gen, whereas stars from other gal­ax­ies could con­tain lots of heav­ier el­e­ments.”

As­tronomers are scratch­ing their heads as to why hy­per­ve­loc­ity stars are rac­ing towards the Ga­lac­tic core

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